Induction of antigen-specific tolerance by administration of the autoantigen is an effective immunotherapy for experimental allergic encephalomyelitis (EAE), a model for inflammatory demyelination of the central nervous system. The mechanisms underlying tolerance induction may include clonal deletion, anergy, and induction of a T-helper1 (Th1) to Th2 phenotype shift. Heterodimers of IL-12p40, IL-12 and IL-23, are proinflammatory cytokines produced by activated antigen-presenting cells (APCs). They have the capacity to drive Th1 differentiation and/or Th1 clonal expansion. We and others have shown that suppression of EAE by intravenous (i.v.) tolerance is associated with downregulation of IL-12, and administration of IL-12 blocks the induction of tolerance. However, mechanisms underlying this phenomenon are not clear. The effect of tolerance on the regulation of IL-23, IL-12/IL-23 receptors, and their signal transduction pathways is not known. Neither is it clear what effect, if any, IL-12/IL-23 have on the function of APCs, which may play a significant role in inducing tolerance through a Th1 to Th2 shift. In this project we will compare and contrast the role of IL-12 and IL-23 in EAE tolerance. Specifically, we will test the hypotheses that 1) i.v. administration of autoantigen results in the generation of APC2 and downregulation of IL-12/IL-23. This, in turn, shifts Th1 towards a Th2 phenotype in tolerized EAE mice; and 2) IL-23 plays a key role in prevention and abrogation of established tolerance. To test these hypotheses, we propose the following Specific Aims: 1) To compare and contrast the role of IL-12 and IL-23 in prevention of induction or abrogation of established i.v. tolerance in EAE; 2) To study the mechanisms of i.v. tolerance-induced regulation of IL-12/IL-23 and APC function in EAE; and 3) To study the effect of i.v. tolerance on the receptors for IL-12 and IL-23 and their signal transduction pathways in EAE. The information gained from these studies should help in defining the mechanisms of i.v. tolerance and pave the way for specific immunotherapies for CNS inflammatory demyelination. [unreadable] [unreadable]